Transcript Phase-Comparison Monopulse.

Advanced
Monopulse Tracking Radar
INTRODUCTION
Typical tracking radars have a pencil
beam to receive echoes from a
single target and track the target in
angle, range, and/or Doppler.
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Its resolution cell—defined by its
antenna beamwidth, transmitter
pulse length (effective pulse length
may be shorter with pulse
compression), and/or Doppler
bandwidth—is usually small
compared with that of a search radar
and is used to exclude undesired
echoes or signals from other targets,
clutter, and countermeasures.
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Electronic beam-scanning phased
array radars may track multiple
targets by sequentially dwelling
upon and measuring each target
while excluding other echo or
signal sources.
Because of its narrow beamwidth,
typically from a fraction of 1° to 1
or 2°, tracking radars usually
depend upon information from a
surveillance radar or other source
of target location to acquire the
target, i.e., to place its beam on or
in the vicinity of the target before
initiating a track.
Scanning of the beam within a
limited angle sector may be
needed to acquire the target
within its beam and center the
range-tracking gates on the echo
pulse prior to locking on the target
or closing the tracking loops.
The gate acts like a fast-acting onoff switch that turns the receiver
"on" at the leading edge of the
target echo pulse and "off at the
end of the target echo pulse to
eliminate undesired echoes. The
range-tracking system performs
the task of keeping the gate
centered on the target echo.
The primary output of tracking
radar is the target location
determined from the pointing
angles of the beam and position of
its range-tracking gates.
The angle location is the data
obtained from synchros and
encoders on the antenna tracking
axes (or data from a beampositioning computer on an
electronic-scan phased array
radar).
In some cases, tracking lag is
measured by converting trackinglag-error voltages from the tracking
loops to units of angle.
This data is used to add to or
subtract from the angle shaft
position data for real-time
correction of tracking lag.
There are a large variety of trackingradar systems. A widely used type of
tracking radar is a ground-based
system consisting of a pencil-beam
antenna mounted on a rotatable
platform with servo motor drive of its
azimuth and elevation position to
follow a target.
C-band monopulse precision tracking radar
Modern requirements for
simultaneous precision tracking of
multiple targets has driven the
development of the electronic scan
array monopulse radar with the
capability to switch its beam pulseto-pulse among multiple targets.
C-band electronic scan phased-array
Multi Object Tracking Radar (MOTR)
The principal applications of
precision tracking radar are
weapon control and missile-range
instrumentation. In both
applications, a high degree of
precision and an accurate
prediction of future position of the
target are generally required.
This lecture describes the
monopulse (simultaneous lobing
with either phase comparison or
amplitude comparison) trackingradar techniques with the main
emphasis on the amplitudecomparison monopulse
(simultaneous lobing) radar.
MONOPULSE
(SIMULTANEOUS LOBING)
The output from the lobes must be
compared simultaneously on a
single pulse, eliminating the effects
of echo amplitude change with time.
This technique was initially called
simultaneous lobing.
Later, the term monopulse was
coined, referring to the ability to
obtain angle error information on a
single pulse. It has become the
commonly used name for this
tracking technique.
The original monopulse tracking radars
suffered in antenna efficiency and
complexity of microwave circuitry
because waveguide signal-combining
circuitry was a relatively new art.
These problems were overcome and
monopulse radar, with modern
compact off-the-shelf processing
circuitry, can readily outperform
scanning and lobing systems.
The monopulse technique also has
an inherent capability for highprecision angle measurement
because its feed structure is
compact with short signal paths
and rigidly mounted with no
moving parts. This has made possible the
development of pencil-beam tracking radars that
meet missile-range instrumentation-radar
requirements of 0.003° angle-tracking precision.
Amplitude-Comparison Monopulse.
A method for visualizing the operation
of an amplitude-comparison receiver
is to consider the echo signal at the
focal plane of an antenna. The echo is
focused to a finite size "spot."
The "spot" is centered on the focal
plane when the target is on the
antenna axis and moves off center
when the target moves off axis.
The antenna feed is located at the
focal point to receive maximum
energy from a target on axis.
The amplitude-comparison feed is
designed to sense any feed plane
displacement of the spot from the
center of the focal plane.
A monopulse feed using the fourhorn square, for example, would
be centered at the focal plane. It
provides symmetry so that when
the spot is centered, equal energy
falls on each of the four horns.
The radar senses target displacement
from the antenna axis that shifts the
spot off of the center of the focal
plane by measuring the resultant
unbalance of energy received in the
four horns.
This is accomplished by use of
microwave waveguide hybrids to
subtract outputs of pairs of horns,
providing a sensitive device that
gives signal output when there is
an unbalance caused by the target
being off axis.
The RF circuitry for four-horn square
feed (see Fig.) subtracts the output
of the left pair from the output of
the right pair to sense any unbalance
in the azimuth direction.
Microwave comparator circuitry used with a four-horn monopulse feed
It also subtracts the output of the
top pair from the output of the
bottom pair to sense any unbalance
in the elevation direction. In
addition, the circuitry adds the
output of all four horns for a sum
signal for detection, monopulse
processing, and range tracking.
The comparator shown in Fig. is the
circuitry that performs the addition
and subtraction of the feed horn
outputs to obtain monopulse sum and
difference signals. It is illustrated with
hybrid-T (or magic-T) waveguide
components. These are four-port
devices that, in basic form, have the
inputs and outputs located at right
angles to each other.
The subtractor outputs are called
difference signals, which are zero
when the target is on axis, increasing
in amplitude with increasing
displacement of the target from the
antenna axis. The difference signals
also change 180° in phase from one
side of center to the other.
The sum of all four-horn outputs
provides a reference signal to
control angle-tracking sensitivity
(volts per degree of error) to
remain constant, even though the
target echo signal may vary over a
large dynamic range.
This is accomplished by automatic
gain control (AGC) to keep the sum
signal output and angle-tracking
loop gains constant for stable
automatic angle tracking.
Block diagram of typical monopulse
radars: The sum-signal, elevation
difference signal, and azimuth
difference signal are each converted to
intermediate frequency (IF), using a
common local oscillator to maintain
relative phase at IF. The IF sum-signal
output is detected and provides the
video input to the range tracker.
Block diagram of a conventional monopulse tracking radar
The range tracker measures and
tracks the time of arrival of the
desired target echo and provides
gate pulses that turn on the radar
receiver channels only during the
brief period when the desired echo
is expected. The gated video is
used to generate the dc voltage
proportional to the magnitude of
the E signal or I EI for the AGC of all
three IF amplifier channels. The AGC
maintains constant angle-tracking
sensitivity (volts per degree error),
even though the target echo signal
varies over a large dynamic range,
by controlling gain or dividing by I E I
AGC is necessary to keep the gain
of the angle-tracking loops
constant for stable automatic angle
tracking.
Some monopulse systems, such as
the two-channel monopulse, can
provide instantaneous AGC or
normalizing by use of log
detectors.
In a pulsed tracking radar, the
angle-error-detector output is
bipolar video; that is, it is a video
pulse with an amplitude
proportional to the angle error and
whose polarity (positive or
negative) corresponds to the
direction of the error.
This video is typically processed by a
sample and hold circuit that charges a
capacitor to the peak video-pulse
voltage and holds the charge until the
next pulse, at which time the capacitor
is discharged and recharged to the
new pulse level. With moderate lowpass filtering, this gives the dc error
voltage output to the servo amplifier
to correct the antenna position.
The three-channel amplitudecomparison monopulse tracking
radar is the most commonly used
monopulse system.
Monopulse-Antenna Feed
Techniques.
Monopulse-radar feeds may have
any of a variety of configurations.
Single apertures are also employed
by use of higher-order waveguide
modes to extract angle-error-sensing
difference signals.
There are many tradeoffs in feed
design because optimum sum and
difference signals, low sidelobe
levels, selectable polarization
capability, and simplicity cannot all
be fully satisfied simultaneously.
The term simplicity refers not only
to cost savings but also to the use
of noncomplex circuitry, which is
necessary to provide a broadband
system with good boresight stability
to meet precision-tracking
requirements.
(Boresight is the electrical axis of the
antenna or the angular location of a
signal source within the antenna
beam at which the angle-errordetector outputs go to zero.)
The original four-horn square
monopulse feed is inefficient because
the optimum feed size aperture for the
difference signals is approximately
twice the optimum size for the sum
signal. Consequently, an intermediate
size is typically used with a significant
compromise for both sum and
difference signals.
The optimum four-horn square feed
is based on minimizing the angle
error caused by receiver thermal
noise. However, if sidelobes are a
prime consideration, a somewhat
different feed size may be desired.
Approach to the ideal - is a 12-horn
feed (Fig.). The overall feed is divided
into small parts, and the microwave
circuitry selects the portions
necessary for the sum and difference
signals to approach the ideal.
Twelve-horn feed
One disadvantage is that this feed
requires a very complex microwave
circuit.
The five-horn feed is selected because
of the simplicity of the comparator that
requires only two magic (or hybrid) T's
for each polarization. The sum and
difference signals are provided for the
two linear-polarization components and
are combined in a waveguide switch for
selecting polarization.
Five-horn feed with coupling to both linear-polarization components, which are
combined by the switch matrix to select horizontal, vertical, or circular polarization
The switch selects either the vertical or the
horizontal input component or combines
them with a 90° relative phase for circular
polarization. This feed does not provide
optimum sum- and difference-signal E
fields because the sum horn occupies
space desired for the difference signals.
The five-horn feed is a practical choice
between complexity and efficiency.
Phase-Comparison Monopulse.
A second monopulse technique is the use of
multiple antennas with overlapping beams
pointed at the target.
Interpolating target angles within the beam
is accomplished by comparing the phase of
the signals from the antennas (for simplicity
a single-coordinate tracker is described).
(a) Wavefront phase relationships in a phase comparison monopulse radar
(b) block diagram of a phase comparison monopulse radar (one angle
coordinate)
If the target were on the antenna
boresight axis, the outputs of each
individual aperture would be in phase.
As the target moves off axis in either
direction, there is a change in relative
phase. The amplitudes of the signals in
each aperture are the same so that the
output of the angle-error phase
detector is determined by the relative
phase (see Fig.).
(a) RF phase-comparison monopulse system with sum and difference
outputs and {b) vector diagram of the sum and difference signals
The phase-detector circuit is
adjusted with a 90° phase shift on
one channel to give zero output
when the target is on axis and an
output increasing with increasing
angular displacement of the target
with a polarity corresponding to
the direction of error.
Typical flat-face corporate-fed
phased arrays compare the output
of halves of the aperture and fall
into the class of phase-comparison
monopulse.
However, the basic signal
processing of amplitude- and
phase-comparison monopulse is
similar, but the control of amplitude
distribution across an array aperture
for the sum and difference signals
maintains efficiency and lower
sidelobes.
The disadvantages of phasecomparison monopulse with separate
apertures compared with amplitudecomparison monopulse are the
relative difficulty in maintaining a
highly stable boresight and the
difficulty in providing the desired
antenna illumination taper for both
sum and difference signals.
The longer paths from the antenna
outputs to the comparator circuitry
make the phase-comparison
system more susceptible to
boresight change due to
mechanical loading (sag),
differential heating, etc.
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